A technique has been proposed for recognizing the positions of left and right white lines on a driving road from an image captured in front of a vehicle by a camera that is mounted on the vehicle (see paragraph [0014] of Japanese Laid-Open Patent Publication No. 11-147473, hereinafter referred to as “JP11-147473A”). According to the technique disclosed in JP11-147473A, in which attention is paid to the fact that the white lines are higher in terms of lightness (brightness) than other portions of the road surface, an image in a range from about 5 [m] to 30 [m] in front of the vehicle is differentiated laterally (horizontally) in order to produce an edge image represented by differential data. In addition, a combination of intervals, at which a differential value corresponding to a change in brightness goes from a positive level to a negative level and in which the intervals fall within a range that makes the intervals acceptable as white lines, is extracted as horizontal data of white line candidates (see paragraphs [0019] through [0021] of JP11-147473A).
Furthermore, a vertical continuity of the white line candidates, as represented by the horizontal data, is verified in the edge image successively from a lower end of the screen, thereby recognizing white lines (see paragraphs [0023] and [0024] of JP11-147473A).